The terms ‘gem’, ‘gemstone’ and ‘stone’ are used interchangeably with the usual meaning referring to minerals such as diamonds, sapphires, rubies, emeralds and so on. In particular cases, without compromising generality, diamonds will be used to describe the invention.
The terms ‘inclusion’, ‘flaw’ and ‘defect’ are used interchangeably indicating an individual visually discernable irregularity inside the gem.
The term ‘dividing plane’ relates to planes through which a stone is sawed, cleaved or cut by any method into separate parts.
Gemstones, as ornamental objects rather than for industrial use, are valued by their appearance. In gemology, the quality of a gem such as a diamond, is typically determined by the “4C's”, Clarity (internal perfection of the stone), Color (colorless being the more expensive), Cut (consisting of shape, proportions, symmetry, and polish), and Carat (weight).
As for clarity, it is desirable to identify the location and size of flaws inside a rough stone in order to determine the preferred dividing planes that would yield the greatest value from a given stone. Likewise it may be desirable to identify flaws in a polished stone in order to determine its value.
In everyday practice a stone is visually examined by experts who try to assess the location and size of the flaws using their experience and following industry rules. Still, it is a human subjective judgment that depends on a particular person's skill and experience and may vary between different individuals and circumstances. Moreover, when a parcel of gemstones is to be evaluated, it could take a long time to assess each stone, so that the parcel value is deduced upon the examination of representative stones only. In uncut gems, it is often virtually impossible to see, much less locate, internal flaws.
To overcome the manual inconsistency and the labor involved, optical methods and devices have been proposed for the detection of flaws in stones. However, the high refractive index of gems, especially diamonds, causes large refractions of incoming and outgoing light and total internal reflections resulting in multiple deflected images of the flaws.
U.S. Pat. No. 4,259,011 describes how to identify the presence of inclusions but not their location. European patent 1,211,503, presents a possible solution for the locating of inclusions in a transparent and at least partially polished diamond by imaging the diamond twice and analyzing the images by computer so as to localize an inclusion with respect to the outer surface of the diamond. Although this patent makes reference to a refractive index correction factor to be included in the computer's calculations, it does not provide a solution to multiple images produced by a single inclusion.
U.S. Pat. No. 4,049,350 teaches eliminating the refractions and reflections at the facets of a cut stone by submerging the stone in a solution of similar refraction index. It describes how to locate an inclusion in a two dimensional plane by aiming a narrow laser beam at a preferred angle to a particular facet.
U.S. Pat. No. 4,152,069 also teaches submerging a cut stone in such a solution and how to find the inclusion within a three dimensional volume.
Neither of the latter references discloses any information as to the medium they used to closely match the refraction index of the gem, this being particularly problematic for diamonds that have a very high refraction index. As far as is known to the present inventors, no such liquid has been suggested in the art for determining flaws in diamonds.
A paper entitled “The Optical Properties of Liquid Selenium” (E. W. Saker, Proc. Phys. Soc. 1952, pp. 785-787) provides some experimental results, including the refraction index of solid and molten selenium in the near infrared region with respect to temperature and wavelength. There is no suggestion in this paper of using this information in any way that is relevant to the problem of determining inclusions in diamonds.
U.S. Pat. No. 4,521,073 teaches an infrared light transmitting fiber produced by a process comprising preparing a core crystalline fiber having a high melting point and a high refractive index, forming around the core fiber a continuous layer of cladding crystal having a low melting point and a low refractive index, and subsequently forming a protective layer on the resulting step-index fiber. The patent teaches the use of Thallium iodide-bromide as the crystal, however, there is no suggestion in the patent of using this information in any way that is relevant to the problem of determining inclusions in diamonds.
Meyrowitz R. teaches materials and compounds having high refractive index in two publications, “Immersion Liquids of High refractive Index”, 99. 746-750 and “A Compilation and Classification of Immersion Media of High Index of Refraction”, pp. 398-409.
The disclosures of all of the above cited references are incorporated herein by reference.